The present invention pertains to write drivers which operate the write heads in mass data storage systems, particularly magnetic data storage systems.
In magnetic data storage systems, a magnetic write head writes or stores data as a sequence of ones and zeros on a magnetic medium, such as a magnetic tape or disc. The write head uses an inductive coil to generate magnetic fields, which form magnetic patterns representing the ones and zeros on the medium. Writing a "one" entails directing electric current through the inductive coil in one direction, and writing a "zero" entails directing current in the opposite, or reverse, direction. Thus, writing specific data requires selectively changing the direction of current through the coil of the write head.
Changing the direction of current through the coil is the function of a write driver. A typical write driver includes an H-switch drive circuit and a control circuit. The H-switch drive circuit, which resembles an "H," has two head pins connected to a write head to form the cross-bar of the H, and four drive transistors: two drive transistors form the upper legs and two form the lower legs of the H. The upper legs connect to a high voltage, and the lower legs connect to a low voltage.
The control circuit, which responds to data signals, selectively operates the four drive transistors as on-off switches, thereby controlling current direction through the write head. Specifically, to direct current left to right through the write head, the control circuit turns on the left-upper and the right-lower drive transistors and turns off the right-upper and the left-lower drive transistors. Conversely, the control circuit turns off the left-upper and right-lower drive transistors and turns on the right-upper and the left-lower drive transistors to direct current right to left through the write head.
Typically, one principal concern in the design of write drivers is speed, or switching rate. Switching rate, a measure of how fast the write driver reverses current direction, defines the spatial transitions between the ones and zeroes written on a magnetic medium, with higher switching rates providing sharper, more distinctive transitions than lower switching rates. Ultimately, a higher switching rate yields closer data spacing and thus greater data capacity for a magnetic medium.
A key determinant of switching rate is head swing voltage, the voltage difference between the head pins of the write driver. Since switching rate is proportional to head swing voltage, a large head swing voltage is desirable. In theory, the maximum head swing voltage, for any write driver, is the difference between the high and low voltages connected to the write driver. However, in typical write drivers, the upper drive transistors (the ones forming the upper legs of the H) impose a lesser practical limit on head swing voltage.
The practical limit stems from two facts: one, the base-emitter pn junctions of these transistors have a low (five-volt) breakdown voltage; and two, most of the head swing voltage appears across these junctions during operation of the write driver. Exceeding the breakdown voltage breaks down these pn junctions, and over time degrades current gain of the upper drive transistors, thereby reducing current to the write head and ultimately the magnetic strength of data it writes to a magnetic medium. Thus, to avoid breaking down the pn junctions of the upper drive transistors, it is necessary to sacrifice switching rate by limiting head swing voltage.
However, even with an appropriately-limited head swing voltage, the junctions of the upper drive transistors are still subject to breakdown. In particular, during reversals of write current direction, the write head exhibits self-inductance, a phenomenon which produces a voltage spike at one of the head pins. The voltage spike shoots several volts above the high voltage and thus may exceed the breakdown voltages of the pn junctions.
Moreover, because of inherent capacitance and inductance in both the drive transistors and the write head, the voltage spike incites a ringing, or oscillating, voltage that last several nanoseconds. Although the ringing voltage eventually decays to a negligible level, its amplitude, based on the originating voltage spike, temporarily exceeds the breakdown voltage of the pn junction, thereby causing the undesirable breakdown of the pn junctions in the upper drive transistors.
Accordingly, there is a need for a write driver that not only prevents breakdown in the upper drive transistors but also allows greater head swing voltage.